Radiation curable compositions for use in coating and printing ink applications have been in commercial use for several years, but, have all generally been lacking in their ability to adhere well to plastic substrates. These compositions, unlike conventional solvent-based or water-based, thermally dried systems, experience high shrinkage during the conversion from their applied monomeric form to their high density, cured, polymeric form. The shrinkage, due to polymerization and crosslinking, can in some instances approach about 30 percent by volume. Furthermore, because of the rapidity of the polymerization, usually less than one second, there is insufficient exposure to heat during the cure, hence the stresses induced in the newly formed polymer cannot relax and a strong bond between the coating and substrate cannot form or persist. It is believed that this unusually large shrinkage and concomitant stresses are mainly responsible for the inadequate adhesion of radiation cured systems to plastic substrates.
Other explanations proposed for the lack of adhesion of photocurable compositions to plastics are insufficient and improper wetting of the substrate surface and an insufficient population of functional groups that can bond or associate with the substrate. The adhesion problem is encountered to a greater extent in the case of most plastic substrates which have a smooth, non-porous surface which surface resists adhesion resulting from interlocking of the coating with surface irregularities.
Early attempts to minimize or rectify the adhesion problem have included the use of adhesion promoting silicones, the inclusion of functional groups in the coating polymer and the use of concurrent or post heat treatment to relieve the polymer stresses during or after polymerization. Only the latter has shown some effectiveness but this post-treatment is not coincident with one of the major objectives of spontaneous radiation curing, that of reduced energy consumption; it also prolongs the cure cycle and, most importantly, it is impractical as it often causes physical damage to the plastic being coated.